Numbering a plurality of wireless sensors coexisting in a communication area and distinguishing the plurality of wireless sensors during monitoring may be generally referred to as a sensor multiple access technology, which has become one of core technologies of wireless sensor networks. Thereby, each measured sensitive variable can be associated with a sensor having a specific number, so as to determine a processing mode, calibration coefficients, a physical location and a measured object and so on for the sensitive variable, which is very beneficial to the monitoring and maintenance of system operation states. Especially for a wireless passive sensor, how to integrate a multiple access function without external power supply is a technical challenge which must be faced with.
There have been a variety of wireless passive sensors with RFID tags in the prior art. The Chinese patent applications No. CN200920108024.3 and No. CN20091008414.1 have disclosed a technology in which a plurality of delayed type reflecting gratings are parallelly connected on a surface acoustic wave wireless sensor. The reflecting gratings of this technology may constitute a plurality of peaks corresponding to a sensor number in a time domain transmitting signal of the sensor. A binary logic value of a bit is determined based on whether a reflection peak exists at a specific moment, which constitutes a multi-bit address code. The limitations of this technology lie in: firstly, as long as a sensor is within a communication of a telemeter, a reflected signal containing an address code and a sensitive variable is formed no matter whether the access is required, which causes conflicts with other similar sensors within the area so that the address and the sensitive variable cannot be determined. Secondly, an address code requires to be realized in a lithography process, which is expensive and cannot be rewritten. As to the above first limitation, a frequency division multiple access or a time division multiple access can be utilized in combination to avoid conflicts between a plurality of sensors, but it leads to an enhanced system complexity and meanwhile a reduced value of address codes. Alternatively, the U.S. Pat. No. 7,952,482B2 and the US application No. 2011/0285510A1 use different frequency orthogonal transmitting gratings, so that an orthogonal or a quasi-orthogonal OFC-PN code address sequence is formed. However, there is still an obvious use limitation, i.e. when a plurality of sensors coexist, clocks of reflected signals for all sensors are required to be strictly synchronized, otherwise a dislocation and overlapping of reflection peaks would appear, so that a sharpness of an autocorrelation function peak for an address code is greatly reduced which causes misreading, and sensitive information cannot be normally extracted. Furthermore, partial code capacity of PN code is sacrificed to ensure the sharpness of the autocorrelation function, so that a code word with a limited length can not be fully utilized.
In an apparatus of the Chinese patent No. CN200780009331.X, an array of sensors are integrated into a RFID modulator. Each sensor is connected to an independent logic gate circuit, so as to constitute a simple analog-digital signal converter. Binary states of these logic gates are inverted with state changes of the connected sensors, constitute a digital pulse sequence in terms of an order, and are reflected to a telemeter as a measuring code, and such a reflected signal is a pure digital communication code signal. The telemeter solves for a state of each sensor according to the measuring code after receiving the measuring code. The limitations of this apparatus lie in: firstly, an on/off state inversion occurs only at a preset threshold of a sensitive variable for each sensor, and a plurality of parallel similar sensors are required and different thresholds of a sensitive variable are set for purpose of fine measurement, which enhances complexity and cost; secondly, a logic gate circuit requires a direct power supply from a RFID power restorer, and the required power is increased with an increase of the monitoring solution and the number of sensitive variables, so that monitoring distance is reduced. In addition, the measuring code and the address code are reflected together in the reflected signal of this apparatus, and when a plurality of sensors of this type coexist, no mechanism for avoiding conflict is provided. Thus, this apparatus can only be utilized for performing telemetry of a sensor existing alone in a communication area, and can not uniquely access each sensor when a plurality of sensors coexist in the area.
The Chinese patent application No. CN201180032851.9 has disclosed a method, which integrates a digital RFID reader with an analog sensor reader, and reduces cost and complexity. The RFID reader reads an address, and an analog impedance monitoring circuit measures a resistance, an inductance or a capacitance at an antenna of sensor. The method may respectively acquire a sensor address and a reflected signal of a sensitive variable at same or different frequencies and at same or different time. However, the method has the following limitations. Firstly, when the analog impedance monitoring circuit works, a plurality of sensors coexisting in a same communication area all reflect analog impedance signals at the same time, which interfere with each other unavoidably and can not be distinguished via the address code. Secondly, only a real part resistance and an imaginary part reactance of a reflected signal impedance can be monitored, and this kind of measurement is influenced by a wavelength periodicity of a wireless signal in a transmission route, which leads to an uncertainty of a phase for the measured complex impedance, thus limiting the monitoring distance, the accuracy and the number of sensitive variables. Therefore, it is only applicable to a near-field coupling application where a relative position between a sensor and a telemeter is fixed rather than an intermediate-remote distance telemetry, and three or less sensors can be externally connected.